First, please try to read the Networking Guide on http://docs.openstack.org. Here is an overview for busy people:

The Network Managers you mention seem to come from the old Nova-Network service, which was replaced by Neutron roundabout 2013. At Ocata, Nova-Network is obsolete.

Neutron is a framework for implementing virtual networking. The actual implementation is done by so-called plugins, which is a hint that there are many possible ways of setting up your network. The most important plugin implements the core resources network, subnet and port and is therefore called the core plugin. Most often, ML2 is used as the core plugin.

ML2 uses so-called type drivers and mechanism drivers to implement virtual networking. Type drivers implement protocols like VLAN or VXLAN. Mechanism drivers implement the wiring of virtual machines to the network; the most popular mechanisms are LinuxBridge and Openvswitch.

There are two categories of virtual networks: Provider networks directly correspond to physical networks in the datacenter. Usually, provider networks use types Flat or VLAN; I don't know if other types are possible. Tenant networks are entirely virtual and can be implemented with network types VLAN, GRE, VXLAN and the less known Geneve. Provider networks can only be set up by the admin, whereas tenant networks can be created and manipulated by users.

A typical config is a single provider network that is also used to connect the cloud to the internet. Such a provider network is named external. A router then connects tenant networks to the external network.